Direct Comparison of Electron Transport and Recombination Behaviors of Dye-Sensitized Solar Cells Prepared Using Different Sintering Processes

Flexible dye-sensitized solar cells on plastic substrates have achieved a conversion efficiency of 8.6% with the hot compression technique (<150 °C). However, the value of efficiency is only 70% of that achieved using glass substrates with high-temperature sintering technique (500 °C). Investigating the origin of this difference is a critical step for further improving the performance of plastic dye-sensitized solar cells. In this study, an optimized ternary viscous titania paste without the addition of organic binders enables the fabrication of efficient dye-sensitized solar cells with a low-temperature process. Therefore, the electron-transport behavior of dye-sensitized solar cells can be directly compared with those prepared with the high-temperature sintering technique. In addition to the structural and optical differences, the hot compressed photoanode of dye-sensitized solar cells have an electron diffusion coefficient that is 2 times smaller and a recombination time that is 6 times shorter than those of the high-temperature sintered cells, suggesting inadequate interparticle connections and more recombination events. These results indicate that electron transport and recombination are still the key factors governing the performance of low-temperature fabricated dye-sensitized solar cells. Eventually, the flexible cell with an efficiency of 6.81% has been achieved on flexible indium tin oxide/polyethylene naphthalate substrate. Further improvements in advanced low-temperature processing or novel materials with minimized defect or grain boundaries are required

Three-Dimensional Optical Tomography and Correlated Elemental Analysis of Hybrid Perovskite Microstructures: An Insight into Defect-Related Lattice Distortion and Photoinduced Ion Migration

Organic lead halide perovskites have recently been proposed for applications in light-emitting devices of different sorts. More specifically, regular crystalline microstructures constitute an efficient light source and fulfill the geometrical requirements to act as resonators, giving rise to waveguiding and optical amplification. Herein we show three-dimensional laser scanning confocal tomography studies of different types of methylammonium lead bromide microstructures which have allowed us to dissect their photoemission properties with a precision of 0.036 μm³. This analysis shows that their spectral emission presents strong spatial variations which can be attributed to defect-related lattice distortions. It is also largely enhanced under light exposure, which triggers the migration of halide ions away from illuminated regions, eventually leading to a strongly anisotropic degradation. Our work points to the need for performing an optical quality test of individual crystallites prior to their use in optoelectronics devices and provides a means to do so

2,2′-Dihydroxy-4,4′-dimethoxy-benzophenon as Bifunctional Additives for Passivated Defects and Improved Photostability of Efficient Perovskite Photovoltaics

Organic–inorganic hybrid perovskite solar cells (PSCs) have developed rapidly in the past decade, but their commercial applications are restricted by further improvement in their photovoltaic performance and stability. Herein, we propose a facile and effective method employing 2,2′-dihydroxy-4,4′-dimethoxy-benzophenon (BP6) as bifunctional additive to construct efficient and photostable PSCs. BP6, as an additive, improves the crystallization quality of perovskite absorbers and further inhibits defect-mediated non-radiative recombination through interaction between the CO group and defects; as a UV absorber, BP6 protects the PSCs from UV degradation by effectively absorbing UV light through molecular tautomerism under continuous strong UV irradiation. Eventually, the champion PSC demonstrates an efficiency of 22.85% with enhanced UV stability after addition of 0.024 wt % BP6. These results reveal that addition of UV absorbers (such as BP6 in this study) is a simple and effective strategy to fabricate efficient and photostable PSCs

Unraveling the Passivation Process of PbI₂ to Enhance the Efficiency of Planar Perovskite Solar Cells

There appears to be a controversy on whether remnant PbI2 is beneficial to the performance of perovskite solar cells (PSCs). We have shown that PSCs with residual PbI2 deposited by one-step antisolvent solution and two-step evaporation-solution method both have shown better performance than those without excess PbI2. X-ray diffraction with diverse X-ray incident angles combined with scanning electron microscopy and secondary-ion mass spectrometry is employed to identify the position of remnant PbI2. It reveals that residual PbI2 is located at grain boundaries near the perovskite/hole-transporting layer interface area for the one-step antisolvent solution method, and the two-step evaporation-solution method situates the excess PbI2 at grain boundaries and the electron transport layer/perovskite interface. The cell performance implies that grain boundary passivation is beneficial for promoting short-circuit current density, while interface passivation is more favorable to enhance open-circuit voltage and fill factor. The revealed passivation process indicates a deep understanding of remnant PbI2 and contributes to the development of PSCs

Efficient and Stable Perovskite Solar Cell Achieved with Bifunctional Interfacial Layers

The elaborate control of the surface morphologies and trap states of solution-processed perovskite films significantly governs the photovoltaic performance and moisture resistance of perovskite solar cells (PSCs). Herein, a thin layer of poly­(triaryl amine) (PTAA) was unprecedentedly devised on top of perovskite quasi-film by spin-coating PTAA/chlorobenzene solution before annealing the perovskite film. This treatment induced a smooth and compact perovskite layer with passivated surface defects and grain boundaries, which result in a significantly reduced charge recombination. Besides, the time-resolved photoluminescence spectra of the PTAA-treated perovskite films confirmed a faster charge transfer and a much longer lifetime compared to the control cells without the PTAA treatment. Moreover, such a hydrophobic polymer atop the perovskite layer could effectively protect the perovskite against humidity and retain 83% of its initial efficiency in contrast to 56% of control cells stored for 1 month in ambient conditions (25 °C, 35 RH%). As a result, the PTAA-treated PSCs displayed an average efficiency of 17.77% (with a peak efficiency of 18.75%), in contrast to 16.15% of the control cells, and enhanced stability. These results demonstrate that PTAA and the method thereof constitute a promising passivation strategy for constructing stable and efficient PSCs

Synergistic Effect of Ammonium Salts in Sequential Deposition toward Efficient Wide-Band-Gap Perovskite Photovoltaics with PCE Exceeding 20%

The controlled crystallization process is of significance to the morphological quality of wide-band-gap perovskite absorbers, especially with excessive bromide ions. Moreover, the non-radiative recombination assisted by surface defects is one of the major unfavorable factors that confines the development of highly efficient wide-band-gap perovskite solar cells (PSCs). Here, 1.65 eV wide-band-gap PSCs are constructed by a sequential deposition method with tailored morphology of highly reproducible perovskite absorbers. The controlled crystallization with the help of NH4Cl enables the perovskite films with larger and more uniform grains, which result in less bulk defects. At the same time, (NH4)2SO4 as a passivation layer reduces the uncoordinated Pb2+ and Pb0 defects on the surface of the perovskite film and improves the hydrophobicity due to newly formed insoluble PbSO4. Eventually, the synergistic effect of ammonium salts results in a high VOC of 1.18 V and an optimal efficiency of 20.43%, which is one of the highest power conversion efficiencies for 1.65 eV wide-band-gap-based PSCs constructed by a two-step deposition process. This work confirms that the sequential deposition method and addition of proper ammonium salts are effective strategies toward highly efficient and stable wide-band-gap PSCs

Delayed Annealing Treatment for High-Quality CuSCN: Exploring Its Impact on Bifacial Semitransparent n‑i‑p Planar Perovskite Solar Cells

Inorganic p-type copper­(I) thiocyanate (CuSCN) hole-transporting material (HTM) belongs to a promising class of compounds integral for the future commercialization of perovskite solar cells (PSCs). However, deposition of high-quality CuSCN films is a challenge for fabricating n-i-p planar PSCs. Here we demonstrate pinhole-free and ultrasmooth CuSCN films with high crystallinities and uniform coverage via delayed annealing treatment at 100 °C, which can effectively optimize the interfacial contact between the perovskite absorber and the electrode for efficient charge transport. A satisfactory efficiency of 13.31% is achieved from CuSCN-based n-i-p planar PSC. In addition, due to the superior transparency of p-type CuSCN HTMs, it is also possible to prepare bifacial semitransparent n-i-p planar PSCs, which eventually permits a maximum efficiency of 12.47% and 8.74% for the front and rear illumination, respectively. The low-temperature process developed in this work is also beneficial for those applications such as flexible and tandem solar cells on heat-sensitive substrates

Annealing-Free SnO₂ Layers for Improved Fill Factor of Perovskite Solar Cells

Perovskite solar cells (PSCs) have developed rapidly with simplified planar structures, in which the electron transport layer (ETL) is one of the key components for high efficiency. As one of the most widely used ETLs for PSCs, a tin dioxide (SnO2) ETL is usually obtained by thermal annealing at around 150 °C, which complicates the fabrication process and confines the application of PSCs onto thermally sensitive flexible substrates. Here, we adopted an annealing-free process for the first time, the negative pressure evaporation (NPE) method, to quickly prepare SnO2 ETLs (NPE-SnO2) within 1 minute at room temperature from widely used commercial aqueous SnO2 colloid. The NPE process developed here significantly improves the surface morphology and conductivity of SnO2 layers compared to the traditional thermally annealed ones (A-SnO2). Detailed characterizations reveal that increased oxygen vacancies and reduced hydroxyl defects contribute to higher conductivity of NPE-SnO2 and less interfacial recombination of PSCs. Therefore, a PSC with NPE-SnO2 delivers an improved fill factor (FF) of 82.33% and a higher power conversion efficiency (PCE) of 23.07%, which is the highest value based on annealing-free SnO2. To conclude, the NPE process is a universal technique to obtain high-quality semiconductor films from their wet state within 1 min and opens up the possibility of fabricating functional layers of PSCs without thermal annealing

1‑Adamantanamine Hydrochloride Resists Environmental Corrosion to Obtain Highly Efficient and Stable Perovskite Solar Cells

Passivating the defective surface of perovskite film is a promising strategy to improve the stability and efficiency of perovskite solar cells (PSCs). Herein, 1-adamantanamine hydrochloride (ATH) is introduced to the upper surface of the perovskite film to heal the defects of the perovskite surface. The best-performance ATH-modified device has a higher efficiency (23.45%) than the champion control device (21.53%). The defects are passivated, interfacial nonradiative recombination is suppressed, and interface stress is released by the ATH deposited on the perovskite film, leading to longer carrier lifetimes and enhancement in open-circuit voltage (VOC) and fill factor (FF) of the PSCs. With obvious improvement, VOC and FF of 1.159 V and 0.796 for the control device are raised to 1.178 V and 0.826 for the ATH-modified device, respectively. Finally, during an operational stability measurement of more than 1000 h, the ATH-treated PSC exhibited better moisture resistance, thermal persistence, and light stability

Defects Healing in Two-Step Deposited Perovskite Solar Cells via Formamidinium Iodide Compensation

Photovoltaics based on metal halide perovskites have recently achieved a certificated efficiency of 25.2%. One of the factors that limit further development of these devices comes from the defective boundaries between crystalline domains in perovskite solar cells (PSCs). Such boundaries represent a significant loss channel causing nonradiative recombination, but systematic optimization procedures have not been developed yet to control their properties. Herein, we propose a facile but effective defect healing method to passivate the defects along the grain boundaries in PSCs by post-treatment of formamidinium iodide (FAI) solution in isopropyl alcohol (IPA). We use a combination of methods including space-charge-limited current, steady-state and time-resolved photoluminescence, confocal laser scanning microscopy, and transient absorption spectroscopy to show the reduction of density of defect states in perovskite films processed with 1 mg/mL FAI solution. The resultant FAI healed PSCs achieve an average power conversion efficiency of 19.26% (with a champion efficiency of 20.62%), higher than that of 16.45% in the control cell. FAI healed devices without encapsulation maintain nearly 95% of the initial efficiency after 60-day storage under N2 environment and nearly 78% of the initial efficiency after 30-day storage under the ambient condition with varied humidity. Our results demonstrate that FAI healing is an effective way to passivate the defect states along grain boundaries for high-efficiency and stable PSCs